Final answer:
To detect fetal heart tones at the end of the first trimester, the nurse should place the Doppler probe just above the symphysis pubis. This location allows for effective auscultation using Doppler-shifted ultrasound, which is a standard part of prenatal care.
Step-by-step explanation:
When a nurse is caring for a pregnant client at the end of the first trimester and wishes to assess for fetal heart tones using a Doppler ultrasound stethoscope, the standard practice is to place the Doppler probe just above the symphysis pubis. At this early stage in pregnancy, the uterus and fetus are still relatively low in the pelvis. Detecting the fetal heartbeat using Doppler-shifted ultrasound is a common and effective diagnostic technique during prenatal care. The fetus's heart rate can be auscultated by placing the Doppler probe in the correct location, which in the first trimester would generally be the lower abdomen, specifically just above the symphysis pubis. As the pregnancy progresses, the location may shift upwards, but at the end of the first trimester, it is typically still too early for the fetus to be high enough to auscultate above the umbilicus.